Thin film resonators (TFRs) are thin film acoustic devices which can resonate in the radio frequency to microwave range, for example, 0.5 to 5 Gigahertz (GHz), in response to an electrical signal. A typical TFR contains a piezoelectric film between a first electrode and a second electrode which supports the electric field across the piezoelectric film when a signal is applied to the device. The piezoelectric film is made of a piezoelectric crystalline material, such as zinc oxide, aluminum nitride (AlN) and other piezoelectric crystalline material, which exhibits a piezoelectric effect. The piezoelectric effect occurs when the piezoelectric material induces a strain in response to an electric field applied across the piezoelectric material, or produces an electric field in response to mechanical stress applied to the piezoelectric material. If the electric field across the piezoelectric film is an alternating electric field having frequency components corresponding to resonant frequencies of the piezoelectric film, the film vibrates at the resonant frequencies (e.g. a fundamental frequency and harmonics), the fundamental frequency of which is approximately defined for a film of uniform thickness as the acoustic velocity (v) in the film divided by two (2) times the thickness (t) of the film or f.sub.r =v/2t. The film mechanically vibrates at the resonant frequencies which in turn produces an alternating electric field at the resonant frequencies.
The physical size of a resonant structure in an electrical circuit is dictated by its operational frequency. For example, the length (L) of a side of a resonant cube is given by L=.lambda./2=v/f, where .lambda. is the wavelength for the wave of frequency f, and v is the wave's velocity in a particular medium. In the microwave region, these wavelengths are on the order of a millimeter, and it is in this region of the electromagnetic spectrum where application of resonant cavities to electrical circuits becomes practical. However, on the scale of today's microelectronics, a single device of millimeter dimensions is comparatively large.
In electronic circuit applications, such materials can be used to form resonant structures, that is, structures which affect a circuit's impedance as a function of a signal's frequency. For example, when embodied as an electrical filter, such a resonant structure will offer low impedance to a desired range of frequencies (the pass band) and high impedance to frequencies outside that range (the stop band). In this way, the device selects the frequencies which pass through the circuit and ideally rejects all others.
Use of piezoelectric materials in the fabrication of microwave resonators offers a potential for tremendous size reduction. The action of a piezoelectric is such that it converts a wave traveling at the speed of light to one traveling at the speed of sound, and vice versa. In most media, the velocity (v) of sound is 1/10,000.sup.th that of light. Thus, referring to the above expression, the wavelength (.lambda.)--and therefore the device dimension--is reduced accordingly.
However, due to the imperfections in the piezoelectric material (polycrystalline structure, grain boundaries, etc.) of the device, waves are also launched parallel to the surface (perpendicular to the bulk mode), producing lateral, or shear, acoustic waves in addition to the acoustic waves propagating perpendicularly. These lateral waves lower the efficiency of these devices. The extra piezoelectric material not directly between the electrodes allows propagation of these shear waves.
Therefore, a method for etching the superfluous portions of a material that permits patterned removal of the material without affecting a surface immediately below it is desired.